As a fixing apparatus used in an electrophotographic image forming apparatus such as a copying machine and a printer, a heat roller type fixing apparatus is frequently used. The heat roller type fixing apparatus includes a pair of rollers (a fixing roller and a pressure roller) pressing each other. Further, both or either one of the pair of rollers internally includes heating means realized by a halogen heater or the like. The heating means heats the pair of rollers to a predetermined temperature (a fixing target temperature). After that, a recording sheet on which an unfixed toner image is formed is carried to a pressure area (a fixing nip area) between the pair of rollers, and then the recording sheet is caused to pass through the pressure area. Thus, a toner image is fixed on the recording sheet due to heat and pressure applied thereto.
Incidentally, a fixing apparatus included in a color image forming apparatus generally uses an elastic roller. The elastic roller is a fixing roller provided with, on its surface, an elastic layer which is made from silicon rubber and/or the like. In the case where the elastic roller is used as the fixing roller, a surface of the fixing roller elastically deforms according to an uneven surface of an unfixed toner image and is in contact with a toner image so as to cover the toner image. This allows a color unfixed toner image whose toner amount is larger than that of a monochrome unfixed toner image to be favorably fixed by using heat. Further, due to strain release of the elastic layer which occurs in a fixing nip area, it is possible to improve a releasing property with respect to color toner, which is more likely to offset than monochrome toner. Furthermore, the fixing nip area has a nip shape protruding upward (i.e., toward the fixing roller side), that is, a so-called inverse nip shape. This makes it possible to more favorably separate a sheet from the fixing roller, thereby allowing the sheet to be separated without using any separation means such as a separation claw (self-stripping). This prevents insufficient image formation which is caused by the separation means.
In order to realize a higher process speed, the fixing apparatus included in such the color image forming apparatus is required to have a greater nip width for the fixing nip area. As means for increasing a nip width, two methods are possible. One is a method of increasing a thickness of the elastic layer of the fixing roller, and the other is a method of increasing a diameter of the fixing roller.
Increasing the thickness of the elastic layer of the fixing roller, however, causes the following problem: The elastic layer has a low heat conductivity. Therefore, in a case where the fixing roller internally includes the heating means as in the conventional arrangement and the fixing roller includes the elastic layer having a large thickness, a temperature of the fixing roller cannot follow an increased process speed due to insufficient heat supply.
On the other hand, increasing the diameter of the fixing roller reduces curvatures of the rollers forming the fixing nip area, thereby increasing the fixing nip area. Increasing the diameter of the fixing roller, however, requires the rollers to increase its heat capacity, thereby causing such a problem that (i) warm-up time is extended and (ii) electric power consumption is increased.
In order to solve these problems, for example, Patent Literature 1: Japanese Patent Application Publication, Tokukaihei, No. 10-307496 (Publication Date: Nov. 17, 1998) discloses a belt type fixing apparatus included in a color image forming apparatus, in which belt type fixing apparatus (i) a heating roller, which is heating means, is provided outside a fixing roller, (ii) a fixing belt is set around the fixing roller and the heating roller, and (iii) the fixing roller and a pressure roller press each other via the fixing belt.
In the belt type fixing apparatus, the fixing belt having a small heat capacity is heated. Therefore, the belt type fixing apparatus provides a short warm-up time. Further, with the belt type fixing apparatus, it is not necessary to integrate a heating source such as a halogen lamp into the fixing roller. This makes it possible to increase a thickness of a low-hardness elastic layer made from sponge rubber and/or the like, thereby securing a large nip width.
Further, for example, a belt type fixing apparatus disclosed in Patent Literature 2: Japanese Patent Application Publication, Tokukai, No. 2002-333788 (Publication Date: Nov. 22, 2002) is a planar heat-generating belt type fixing apparatus in which a heat-generating body is used as heating means. In the planar heat-generating belt type fixing apparatus, (i) the heating means has a smaller heat capacity than that of a conventional heating roller, and (ii) a heating member, forming a planar heat-generating body (heating means), itself generates heat. This improves heat responsiveness, compared with the conventional fixing apparatus in which the heating roller is heated indirectly by means of the halogen lamp. This attains (i) a further reduction in warm-up time and (ii) further energy saving.
The conventional planar heat-generating belt type fixing apparatus, however, has the following problems: In a case where small-size sheets whose widths are smaller than a maximum sheet passing width are fed in succession, an area where the small-size sheets has passed is heated by the heating member so as to recover heat lost from the area, thereby returning to its original temperature. On the other hand, regardless of the fact that no heat is lost from a non-sheet passing area which is outside the area where the small-size sheets have passed, the non-sheet passing area is heated by the heating member. This increases a temperature of the non-sheet passing area excessively. This may cause a deterioration of the fixing belt or the fixing roller, or may cause a high-temperature offset when a regular-size sheet is fed immediately after the small-size sheets are fed.
Patent Literature 2 deals with this problem by dividing a system into (i) a system for generating heat only in a center area (when viewed in a longitudinal direction of the heat-generating body) of the heat-generating body and (ii) a system for generating heat only in side areas (when viewed in the longitudinal direction of the heat-generating body) of the heat-generating body. This arrangement, however, requires temperature sensors (such as thermistors) and safety switches (such as thermostats) as much as the number of systems thus divided. With this arrangement, the system becomes very complicated.
Incidentally, Patent Literature 3: Japanese Patent Application Publication, Tokukaihei, No. 5-19652 (Publication Date: Jan. 29, 1993) discloses a technique for preventing a temperature increase in a non-sheet passing area in such a manner that an electrode is formed by using, as a heating body, a self-temperature-control heat-generating body having positive temperature coefficient (PTC) characteristics and thereby current is caused to flow in a direction in which a heat-resistant film (fixing belt) moves. However, as disclosed in Patent Literature 3, a heat-generating body having the PTC characteristics at a high temperature of 200° C. or higher can only be the one made from ceramic materials (sintered moldings) such as a barium titanate. It is difficult to process these materials into the one whose shape corresponds to a shape of a planar heat-generating body, which has a curvature and a large width, as seen in the planar heat-generating belt type fixing apparatus.